Having used 6-7 of them, I would say it depends greatly on where you live, if you travel alot, and what your purpose for use of the phone is. Some carriers have a much greater coverage area than others, some have a smaller area but possibly less dropped calls, and of course, they all have their different "calling plans" and price structures.
It's really something where you need to compare what they offer with what your specific needs are and then try out perhaps their pay by month plan which 2-3 of them offer ("Prepaid," I think it is called). Then if you do not like the service at all, you can switch without battling with them over a 1-2 year contract termination fee.

What's important to you? Coverage? Price? Dropped-calls? Clarity? Benefits?
The thing is they all have their ups and downs. Some offer you free nights and weekends.
Cingular offers free calling to other Cingular customers on most of it's plans, including pre-paid. If you know a bunch of people with Cingular it might be the way to go. They also have nationwide roaming and long distance included.
T-Mobile has the best overall prices, I think, but I hear that their coverage and call quality is lacking in some areas. If price is the deal breaker, then you might stick with them.
Verizon supposedly has the fewest dropped calls, but their prices and phones can be a bit pricey at times.
One more thing to point out about Cingular is that they just merged with AT&T- so you might want to wait and see if they bring any changes to their service before considering them as an alternative.
You said you already have T-Mobile. Are you happy with them so far? I always say, "If it ain't broke, don't fix it."

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